Normal position of the aqueduct of Sylvius. Part 1: significance of aqueduct to dorsum sellae measurements.

The author describes a method for estimating the position of the aqueduct of Sylvius based on the ratio of the measured distance from surface to aqueduct (A) to that from the surface to dorsum sellae (B). The normal ratio A:B is 0.72 with a range from 0.68 to 0.75. Values below 0.68 indicate a dorsally displaced aqueduct.